When a pair of gears are intermeshed in a drive train assembly, every gear tooth along the face of the respective gears is not in exact increment and/or alignment with corresponding gear teeth. Standard clearances and manufacturing tolerances permit small variations in gear tooth size, pitch and the like. This in turn causes the interacting surfaces of meshing gears to exhibit small gaps between their respective meshing teeth which can often permit the entrance of a limited amount of slack or backlash into the system. In applications such as robotic manipulators where accurate and steady movement is often critical to successfully accomplishing the task at hand, even a relatively small amount of such backlash can be very harmful. Moreover, where a series of gears and/or where several gear trains are connected together in one way or another, any such backlash in the system can become additive and cyclical. For example, where a series of gears are utilized to create rotational movement of a portion of a robotic manipulator, such backlash may cause the rotational movement to vary several degrees, and such variance may not necessarily be predictable. If the robotic manipulator is rotating an extended arm or the like, several degrees variance can become quite a substantial alignment problem at the distal end of the extended arm, thereby making the robotic manipulator much less accurate and, possibly, inappropriate for delicate operations.
The industry has attempted to control or eliminate gear backlash in various ways through the years. For example, two separate gear housings are sometimes used, utilizing timing belts to syncronize the drive gear inputs into a single driven gear. This arrangement depends upon the belts in the system to resiliently absorb any backlash which may be present. An alternate method used to minimize backlash problems is discussed in U.S. Pat. No. 4,403,907, which issued to F. H. Koller, et al. on Sept. 13, 1983. In particular, Koller et al. suggests that in order to eliminate unwanted axial backlash in their material handling apparatus, direct axial loading of the main guide shaft by a toggle linkage assembly is utilized. The toggle linkage assembly is designed to provide axial loading of the main guide shaft while permitting rotational movement about the axis of the main guide shaft.
In another robotic application where accurate registration of a rotating swing arm is required for a pick and place function, U.S. Pat. No. 4,398,863, which issued to L. Y. Shum on Aug. 16, 1983, teaches the use of an idler sprocket which can be adjusted to insure proper tensioning of the drive chain therewithin. While the Shum reference states that such an idler sproket can be used to "promote the elimination of backlash", it also incorporates the use of shot pins mounted on the swing arm which are designed to engage hardened bushings to insure proper indexing of the swing arm at each pick and place point location. In many robotic manipulator applications, however, the use of shot pins and bushings is not practical.
Consequently, despite all of the prior work done in this area, there remain problems of effectively and efficiently eliminating unwanted backlash commonly encountered in gear trains and the like in robotic manipulator applications. With prior art mechanisms, it was necessary to utilize inefficient timing belts, adjustable idler sprockets, shot pin and bushing interactions, or direct loading of moveable portions of the device. In modern robotic applications, the prior systems simply do not provide reasonable solutions for eliminating backlash in robotic manipulators designed to accomplish relatively critical location movements in a predictable and reliable manner.